Contactless reader devices as defined in the opening paragraph are widely used nowadays, in particular in the service sector, in the field of logistics, in the field of commerce, and in the field of industrial production. Examples generally based on smart cards are access systems to buildings and offices, payment systems, and smart cards for identifying individuals (e.g. passports). Examples based rather on the so-called RFID tags are systems for identification/pricing of objects and products, or item management in general (this technique is often referred to as electronic bar code, EPC for short), animal tagging, and paper with RFID tags inside. One will appreciate, of course, that the above examples merely represent a small segment of the numerous applications and serve to illustrate what smart cards and RFID tags may be used for. In addition, the upcoming technology of Near Field Communication, NFC for short, also belongs to the same technical field. In the following, RFID tags and smart cards as well as NFC devices, which operate in a passive mode, are generally referred to as transponders.
Reader devices are needed for all these systems (except for NFC, as the devices may also act as readers), which are to communicate with transponders within the radio range. Therefore, a reader device sends out radio signals, which may comprise commands or data and which can be received by a transponder. In addition, data and commands can be sent back to the reader by the transponder, where a distinction is to be made between passive and active transponders. A passive transponder uses the energy of the electromagnetic field to power itself, whereas an active transponder has its own power source, e.g. a battery. Furthermore, different kinds of coupling between a reader device and a transponder have to be distinguished. There is inductive coupling, electromagnetic backscatter coupling, close coupling, and electrical coupling. Finally, the various systems operate in different frequency ranges, starting from a few kHz up to several GHz. For the sake of brevity, reference is only made to general system characteristics below. However, one will easily perceive that the object of the invention and the measures taken to achieve this object are applicable to all kinds of identification systems.
One problem to be overcome in designing a reader/transponder system (e.g. an RFID system) is the question of how to handle a multitude of transponders within the radio range at the same time, i.e. how to collect data from the transponders such that data from one transponder is not superposed by data from another transponder, which is commonly denoted “data collision”? These collisions are a result of the fact that in the beginning of a so-called “inventory”, in which data from all transponders, normally the ID-numbers of transponders, are collected by a reader device, neither the reader device nor the transponders have information on how many transponders are within the radio range of the reader device. This number is revealed step by step during an iterative reading procedure, which will be explained below with reference to FIGS. 1, 2a, and 2b. A similar example can be found in “Specification for RFID Air Interface—EPC Global”, Version 1.0.9, 2004, EPC Global Inc., in particular section 6.3.2 and Annex B, C and F. A further similar example can be found in “Technical Report—13.56 MHZ ISM Band Class 1 RFID Interface Specification”, version 1.0.0, 2003, Audio-ID Center, in particular section B. Finally, ISO/IEC 18000-6 Type A represents yet another method based on time slots, whose multiple seizure by transponders leads to a data collision.
FIG. 1 now shows an arrangement of a reader device RD with four transponders T1 . . . T4 within the radio range. To read data DAT1 . . . DAT4 from the transponders T1 . . . T4, the reader device RD sends out a read command INV, which is received and subsequently processed by the transponders T1 . . . T4. As a result, the requested data DAT1 . . . DAT4 are transmitted from the transponders T1 . . . T4 to the reader device RD.
FIG. 2a shows a prior art timing diagram of a read sequence CYC1 of the arrangement shown in FIG. 1, which read sequence CYC1 consists of eight separate time slots TS1 . . . TS8. Said time slots TS are shown in the first row of the timing diagram. The second row shows data or commands transmitted from the read device RD to the transponders T1 . . . T4. The third to sixth rows show data that are sent back from the transponders T1 . . . T4 to the read device RD. The function of the arrangement of FIG. 1 is now as follows:
First of all a read command INV (also termed “inventory”, “query”, “begin round”, or “init round”) is issued by the reader device RD. This read command INV also comprises an item of information on how many time slots TS are to be used by the transponders T1 . . . T4 in sending back data. Hence, the number N of time slots TS is included in the read command INV, in the present example the number N=8. This number N can be determined by the reader device RD strictly randomly, based on an initial setting, or based on earlier experience, i.e. the reader device RD determines the number N from an adaptive algorithm.
Said read command INV with the number N is now received by the transponders T1 . . . T4. Based on this number N, the transponders T1 . . . T4 determine at which point in time, i.e. in which time slot TS they will answer. Commonly, this is done by randomly selecting one from all time slots TS, here by selecting one of the eight time slots TS1 . . . TS8. In the present example, the first transponder T1 chooses the first time slot TS1 to send back data DAT1 to the reader device RD. The second transponder T2 also chooses the first time slot TS1. The third transponder T3 chooses the fourth time slot T4 and the fourth transponder T4 finally the sixth time slot TS6. Now the data transmission may start.
The first timeslot TS1 starts shortly after receiving the read command INV, and hence the first and the second transponder T1 and T2 start transmitting their data DAT1 and DAT2. Unfortunately a reader device RD is usually not able to distinguish between the data streams, and therefore the data DAT1 and DAT2 are not received correctly. This state is usually called a “data collision”, meaning that more than one transponder T1 . . . T4 sends its data DAT1 . . . DAT4 back to a reader device RD at the same time. However, if frequency multiplexing or code multiplexing is used, data can be received simultaneously by more than one transponder T1 . . . T4, as will be explained further below.
The reader device RD detects this data collision and sends a so-called “close slot” command CS to the transponders T1 . . . T4, which means that the next, here the second timeslot TS2, may start. It should be noted that the “close slot” command CS may appear in different techniques or standards under different names but with the same function, i.e. to inform the transponders T1 . . . T4 within the radio range of the reader device RD that the next time slot TS starts.
In the second and in the third time slot TS2 and TS3, no transponder T1 . . . T4 responds. Hence, the reader device RD switches to the next time slot by means of a close slot command CS after a comparatively short time, thus accelerating the read sequence CYC1. Therefore, the reader device RD waits during a waiting time for a transponder T1 . . . T4 to start transmitting data DAT1 . . . DAT4. After this waiting time the reader device RD proceeds with issuing the close slot command CS in the case in which no data DAT1 . . . DAT4 are received.
In the fourth time slot TS4, the third transponder T3 sends back its data DAT3. Here there is no further transponder T1, T2, T4 transmitting its data DAT1, DAT2, DAT4 back to the reader device RD so that here, contrary to the first time slot T1, no data collision occurs. The third data DAT3 is therefore correctly transmitted to the reader device RD. Therefore, the reader device RD responds with a so-called “close slot and quiet” command CQ (also termed “fix slot” or “next slot”), which marks a switch to the next time slot as explained above and in addition sets the addressed transponder, here the third transponder T3, in a quiet state. This quiet state means that the transponder stays quiet if it receives another read command INV until it is powered down. Again it has to be noted that equal commands and procedures may have different names or even slightly different causes in different standards or different techniques. Those skilled in the art may easily apply the invention also to those standards and techniques.
After the fourth time slot TS4, a further, empty time slot follows, namely the fifth time slot TS5. After that the fourth transponder T4 transmits its data DAT4 back to the reader device RD within the sixth time slot TS6. This sixth time slot TS6 is closed by a close slot and quiet command CQ as explained above. Finally, there are two more empty time slots, namely the seventh and the eighth ones TS7 and TS8, after the first read sequence CYC1 has been completed.
Since the read device RD has detected a data collision in the first read sequence CYC1, it is clear that data DAT1 . . . DAT4 were not received from all transponders T1 . . . T4. Hence, the reader device RD starts a second read sequence CYC2 by issuing another read command INV as shown in FIG. 2b. Since a plurality of empty time slots TS were detected in the first read sequence CYC1, the reader device RD now decides to reserve only six time slots TS. Hence, the number N comprised in the read command INV is set to N=6 (note that in common systems the number N is normally limited to powers of 2, that is to say 2x for natural numbers x. Although another number N was chosen for the present example, this does not mean that the invention does not apply also to systems with such a limited possibility of choice). The procedure is equal to the one explained above for the timing diagram in FIG. 2a. Here three time slots TS1 . . . TS3 are empty before data DAT1 from the first transponder T1 and subsequently data DAT2 from the second transponder T2 are transmitted to the reader device RD in the fourth and fifth time slots TS4 and TS5. The sixth time slot TS6, finally, is empty again.
No data collision was detected in the second read sequence CYC2, so that it is clear that data DAT1 . . . DAT4 have been received from all transponders T1 . . . T4. One can easily recognize that much time is wasted by empty timeslots TS and multiple transmissions of a read command INV. This results in a correspondingly long running time of the read procedure, which is simply the sum of the time of the first read sequence t1 and the time of the second read sequence t2. Hence, it is an object of the invention to shorten the reading procedure.